[PATCH V2] internal-fn: Support undefined rtx for uninitialized SSA_NAME

[Juzhe-Zhong <juzhe.zhong@rivai.ai>]

According to PR: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110751

As Richard and Richi suggested, we recognize uninitialized SSA_NAME and convert it
into SCRATCH rtx if the target predicate allows SCRATCH.

It can help to reduce redundant data move instructions of targets like RISC-V.

gcc/ChangeLog:

	* internal-fn.cc (expand_fn_using_insn): Support undefined rtx.
	* optabs.cc (maybe_legitimize_operand): Ditto.
	(can_reuse_operands_p): Ditto.
	* optabs.h (enum expand_operand_type): Ditto.
	(create_undefined_input_operand): Ditto.

---
 gcc/internal-fn.cc |  4 ++++
 gcc/optabs.cc      | 16 ++++++++++++++++
 gcc/optabs.h       | 14 +++++++++++++-
 3 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/gcc/internal-fn.cc b/gcc/internal-fn.cc
index 0fd34359247..61d5a9e4772 100644
--- a/gcc/internal-fn.cc
+++ b/gcc/internal-fn.cc
@@ -247,6 +247,10 @@ expand_fn_using_insn (gcall *stmt, insn_code icode, unsigned int noutputs,
 	create_convert_operand_from (&ops[opno], rhs_rtx,
 				     TYPE_MODE (rhs_type),
 				     TYPE_UNSIGNED (rhs_type));
+      else if (TREE_CODE (rhs) == SSA_NAME
+	       && SSA_NAME_IS_DEFAULT_DEF (rhs)
+	       && VAR_P (SSA_NAME_VAR (rhs)))
+	create_undefined_input_operand (&ops[opno], TYPE_MODE (rhs_type));
       else
 	create_input_operand (&ops[opno], rhs_rtx, TYPE_MODE (rhs_type));
       opno += 1;
diff --git a/gcc/optabs.cc b/gcc/optabs.cc
index 32ff379ffc3..d8c771547a3 100644
--- a/gcc/optabs.cc
+++ b/gcc/optabs.cc
@@ -8102,6 +8102,21 @@ maybe_legitimize_operand (enum insn_code icode, unsigned int opno,
 	  goto input;
 	}
       break;
+
+    case EXPAND_UNDEFINED:
+      {
+	mode = insn_data[(int) icode].operand[opno].mode;
+	rtx scratch = gen_rtx_SCRATCH (mode);
+	/* For SCRATCH rtx which is converted from uninitialized
+	   SSA, we convert it as fresh pseudo when target doesn't
+	   allow scratch rtx in predicate. Otherwise, return true.  */
+	if (!insn_operand_matches (icode, opno, scratch))
+	  {
+	    op->value = gen_reg_rtx (mode);
+	    goto input;
+	  }
+	return true;
+      }
     }
   return insn_operand_matches (icode, opno, op->value);
 }
@@ -8147,6 +8162,7 @@ can_reuse_operands_p (enum insn_code icode,
     case EXPAND_INPUT:
     case EXPAND_ADDRESS:
     case EXPAND_INTEGER:
+    case EXPAND_UNDEFINED:
       return true;
 
     case EXPAND_CONVERT_TO:
diff --git a/gcc/optabs.h b/gcc/optabs.h
index c80b7f4dc1b..4eb1f9ee09a 100644
--- a/gcc/optabs.h
+++ b/gcc/optabs.h
@@ -37,7 +37,8 @@ enum expand_operand_type {
   EXPAND_CONVERT_TO,
   EXPAND_CONVERT_FROM,
   EXPAND_ADDRESS,
-  EXPAND_INTEGER
+  EXPAND_INTEGER,
+  EXPAND_UNDEFINED
 };
 
 /* Information about an operand for instruction expansion.  */
@@ -117,6 +118,17 @@ create_input_operand (class expand_operand *op, rtx value,
   create_expand_operand (op, EXPAND_INPUT, value, mode, false);
 }
 
+/* Make OP describe an undefined input operand for uninitialized
+   SSA.  It's the scratch operand with mode MODE; MODE cannot be
+   VOIDmode.  */
+
+inline void
+create_undefined_input_operand (class expand_operand *op, machine_mode mode)
+{
+  create_expand_operand (op, EXPAND_UNDEFINED, gen_rtx_SCRATCH (mode), mode,
+			 false);
+}
+
 /* Like create_input_operand, except that VALUE must first be converted
    to mode MODE.  UNSIGNED_P says whether VALUE is unsigned.  */
 
-- 
2.36.3